Legal Question in Consumer Law in Illinois

I would say yes. A public swimming pool can tell you that you cannot have glass bottles, for example, at the pool. For safety reasons, it would seem a reasonable restriction to limit how many tennis balls are on a court at one time. Not sure what you mean by "giving advice . . as not for profit." Sounds like a euphemism for giving lessons. Without more information I am reluctant to give an opinion, but I can see that the owner of a tennis court would be able to limit who give lessons on its court in most circumstances.